In the production of a solid electrolytic capacitor, as shown in FIG. 1, an oxide dielectric film layer 2 is generally formed on an anode substrate 1 comprising a metal foil subjected to an etching treatment to have a large specific surface area, a solid semiconductor layer (hereinafter referred to as a “solid electrolyte”) 4 is formed as a counter electrode in the outer side of the dielectric layer and further thereon, an electrically conducting layer 5 such as electrically conducting paste is preferably formed, thereby fabricating a capacitor basic device. This device by itself or a stacked body resultant from stacking these devices is connected with lead wires 6,7 and thereafter, the whole is completely molded with epoxy resin 8 or the like and then put into use as a capacitor part in electric articles over a wide range.
In recent years, with the progress of digitization of electrical instruments or high-speed processing of personal computers, a compact and large-capacitance capacitor or a capacitor showing low impedance in the high frequency region is being demanded.
As the compact and large-capacitance capacitor, electrolytic capacitors such as aluminum electrolytic capacitor and tantalum electrolytic capacitor are known.
The aluminum electrolytic capacitor is advantageous in that a large-capacitance capacitor can be obtained at a low cost but suffers from such problems that when an ion conducting liquid electrolyte is used as the electrolyte, the impedance in the high frequency region is high, the capacitance decreases accompanying the evaporation of the electrolytic solution with the passing of time and the temperature characteristics are inferior.
The tantalum electrolytic capacitor where a manganese oxide is generally used as the electrolyte, has such problems that the manganese oxide predominantly produced by the thermal decomposition of manganese nitrate cannot be evaded from the possibility of the dielectric film having damages at the thermal decomposition and due to the relatively high specific resistance, the impedance in the high frequency region is high.
In order to solve these problems, it has been proposed to use an electrically conducting polymer having electric conductivity as the solid electrolyte. For example, use of an intrinsic conducting polymer having an electric conductivity of 10−3 to 103 S/cm (see, JP-A-1-169914 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) (corresponding to U.S. Pat. No. 4,803,596)) and use of a polymer such as polyaniline (see, JP-A-61-239617), polypyrrole (see, JP-A-61-240625), a polythiophene derivative (see, JP-A-2-15611 (corresponding to U.S. Pat. No. 4,910,645)) or polyisothianaphthene (see, JP-A-62-118511) are known. These electrically conducting polymers comprising a π-conjugated structure are mostly used in the form of a composition containing a dopant.
In recent years, not only the addition of a dopant but also a combination use with, for example, manganese dioxide (see, JP-B-6-101418 (the term “JP-B” as used herein means an “examined Japanese patent publication”) (corresponding to U.S. Pat. No. 4,959,753)) or filler (see, JP-A-9-320901) is employed.
With respect to the shape of the solid electrolyte, it has been proposed to weld a metal onto an aluminum foil and thereby form a starting point for growing an electrically conducting polymer by the electrolytic oxidative polymerization throughout the surface of the aluminum foil (see, JP-A-4-307917).
Also, a method of performing the alternate impregnation with a monomer solution and with an oxidizing agent solution each from 1 to 20 times and the dipping in an oxidizing solution for 5 minutes to 5 hours, thereby improving the polymerization efficiency, has been proposed (see, JP-A-11-238648).